1. Field of the Invention
The present invention relates to diplexers for use in mobile phones, and high-frequency switches and antenna duplexers using the diplexers. More specifically, the present invention relates to a diplexer for a wireless communications device, such as a mobile phone, capable of using a plurality of communications schemes of different frequency bands, and a high-frequency switch and an antenna duplexer using the diplexer.
2. Description of the Background Art
Mobile phone systems use various schemes, such as EGSM, DCS, UMTS, and PCS schemes. The EGSM (Enhanced-Global System for Mobile Communication) scheme is available mainly in Europe. The DCS (Digital Cellular System) scheme using a 1.8 GHz frequency band has become widely available with the expansion of users. The UMTS (Universal Mobile Telecommunications System) scheme has the potential to achieve next-generation high speed communications. The PCS (Personal Communications Services) system using a 1.9 GHz frequency band is available mainly in the United States.
FIG. 19 is an illustration of frequency bands used in the EGSM, DCS, and UMTS schemes. In FIG. 19, bands denoted by “Tx” in parentheses under schemes are frequency bands for use in transmission, and bands denoted by “Rx” in parentheses under schemes are frequency bands for use in reception. As illustrated in FIG. 19, the EGSM scheme uses 880 to 960 MHz, the DCS scheme uses 1710 to 1880 MHz, and the UMTS scheme uses 1920 to 2170 MHz. As evident from FIG. 19, the mobile phone systems can be broadly divided into systems using the low frequency band, such as the EGSM scheme, and systems using a high frequency band, such as the DCS and UMTS schemes.
In recent years, with the expansion of users of mobile communications devices such as mobile phones and the globalization of mobile communications systems, it is desired to develop mobile phones capable of using a plurality of schemes as mentioned above, such as using the conventionally-available EGSM scheme of the low frequency band and the recently-available DCS and UMTS schemes of the high frequency band. Such advanced mobile phones should be capable of carrying out multiplex and demultiplex signals in a low-frequency band and signals in a high-frequency band in their internal circuit, because different power amplifiers are required for signals of different frequency bands.
Conventionally, to multiplex and demultiplex signals of the low frequency band and the high frequency band, a device called diplexer has been used. FIG. 20 is a diagram illustrating an equivalent circuit of a conventional diplexer (for example, refer to Japanese Patent Laid-Open Publication No. 2000-349581, paragraph [0003], FIG. 6). In FIG. 20, the conventional diplexer includes a low-pass filter (hereinafter, LPF) 10 connected between a first terminal P51 and a second terminal P52, and a high-pass filter (hereinafter, HPF) 20 connected between the first terminal P51 and a third terminal P53.
In the LPF 10, a first inductor L51 and a first capacitor C51 are placed in parallel between the first terminal P51 and the second terminal P52, a second capacitor C52 is placed in series between the second terminal P52 and a ground. The LPF 10 passes signals of the low frequency band in the EGSM scheme, for example.
In the HPF 20, a third capacitor C53 and a fourth capacitor C54 are placed in series between the first terminal P51 and the third terminal P53. A second inductor L52 and a fifth capacitor C55 are placed in series between a point of connection of the third capacitor C53 and the fourth capacitor C54 and the ground. The HPF 20 passes signals of the high frequency band in the DCS or UMTS scheme, for example.
In the LPF 10, the first inductor L51 and the first capacitor C51 form a parallel resonant circuit, which has a predetermined constant so as to resonate with signals of the high frequency band in the vicinity of a passband of the HPF 20. When the parallel resonant circuit resonates, its impedance is significantly increased, thereby preventing the LPF 10 from passing signals of the high frequency band. That is, the LPF 10 provides an attenuation pole in the high-frequency band. Here, the attenuation pole is a position where a specific frequency is greatly attenuated in an attenuation band of a filter.
FIG. 21 is a graph illustrating transmission characteristics of the LPF 10. A dotted curve indicates transmission characteristics of the LPF 10 on the scale of a right vertical axis in the graph. As illustrated in FIG. 20, the LPF 10 provides an attenuation pole AP50 in the frequency band of the UMTS scheme, and passes signals best in the frequency band of the EGSM scheme.
In the HPF 20, the second inductor L52 and the fifth capacitor C55 form a serial resonant circuit, which has a predetermined constant so as to resonate with signals in the frequency band of the EGSM scheme (frequencies corresponding to the vicinity of a passband of the LPF 10). When the serial resonant circuit resonates, its impedance is significantly decreased to effectively become zero. The impedance from the first terminal P51 to the third terminal P53 is significantly increased compared with the LPF 10, leading to the HPF 20 being prevented from passing signals of frequencies corresponding to the EGSM scheme. That is, the HPF 20 provides an attenuation pole in the vicinity of the frequency band of the EGSM scheme.
FIG. 22 is a graph illustrating transmission characteristics of the HPF 20. A dotted curve indicates transmission characteristics of the HPF 20 on the scale of a right vertical axis in the graph. As illustrated in FIG. 22, the HPF 20 provides an attenuation pole AP51 in the frequency band of the EGSM scheme, and passes signals best in the frequency band of the UMTS scheme.
In the above-described conventional diplexer, a signal in the frequency band of the EGSM scheme supplied to the first terminal P51 is almost completely prevented from passing through the HPF 20, but is able to pass through the LPF 10 almost completely. Therefore, such a signal is transmitted not to the HPF 20 side but to the LPF 10 side, and is extractable from the second terminal P52. On the other hand, a signal of the high frequency band supplied to the first terminal P51 is almost completely prevented from passing through the LPF 10, but is able to pass through the HPF 20 almost completely. Therefore, such a signal is transmitted not to the LPF 10 side but to the HPF 20 side, and is extractable from the third terminal P53. As such, with the use of the conventional diplexer, low-frequency signals and high-frequency signals can be multiplexed and demultiplexed.
As illustrated in FIG. 21, in the LPF 10, the signal in the frequency band of the UMTS scheme is sufficiently attenuated and therefore cannot pass through the LPF 10. In contrast, the signal in the frequency band of the DCS scheme is not sufficiently attenuated and therefore part of the signal can pass through the LPF 10. Consequently, as illustrated in FIG. 22, the part of the signal in the frequency band of the DCS scheme passing through the LPF 10 is attenuated to pass through the HPF 20. This poses a problem for a mobile phone supporting both the DCS scheme and the UMTS scheme.
As such, in the above-described diplexer, the LPF 10 placed between the first terminal P51 and the second terminal P2 provides the attenuation pole 50 in the vicinity of the passband of the HPF 20 by using a single parallel resonant circuit. This makes it difficult to ensure attenuation over a wide frequency band. As a result, in the conventional diplexer, the transmission characteristics of the HPF 20 become narrow, making it difficult for signals of the high frequency band to be used in a plurality of mobile phone systems to pass without being attenuated over a wide frequency band.